Optical fiber and optical fiber wiring board using the optical fiber

ABSTRACT

Optical fibers ( 220 ) are routed over a substrate ( 210 ), and an optical fiber wiring board ( 200 ) is thereby constructed. A relative refraction index difference Δ of the optical fiber ( 220 ) is increased to be greater than a relative refraction index difference Δ 0  of a communication-dedicated single mode optical fiber, and a core diameter thereof is increased to be larger than a core diameter of the communication-dedicated optical fiber. Thereby, a mode field diameter thereof is set to be substantially the same as a mode field diameter of the communication-dedicated optical fiber.

TECHNICAL FIELD

[0001] The present invention relates to an optical-fiber wiring boardand an optical-fiber wiring assembly for optically interconnectingoptical elements, optical circuits or optical apparatuses, and moreparticularly to an optimal optical fiber for the optical-fiber wiringboard.

BACKGROUND ART

[0002] Conventionally, an optical-fiber wiring board is used tooptically interconnect optical elements, optical circuits or opticalapparatuses. The optical-fiber wiring board is constructed to include aplurality of optical fibers routed on a substrate in a predeterminedpattern. Generally, a communication-dedicated single mode optical fiberhaving an outside diameter of 250 μm is used as an optical fiber that isrouted on the optical-fiber wiring board. For example, JapaneseUnexamined Patent Publication No. 2000-66035 discloses an optical fiberhaving an outside diameter of 125 μm for an optical-fiber wiring board.

[0003] In the optical-fiber wiring board, a number of portions in whichthe optical fibers are bent and routed. As such, the bending loss of theoptical fibers is increased, and the performance of the optical-fiberwiring board is thereby lowered. Hence, for the optical-fiber wiringboard, the optical fibers need to be routed not to increase the bendingloss. However, a problem arises in that such a routing manner makes itdifficult to implement the miniaturization of the optical-fiber wiringboard.

[0004] In more specific, on the optical-fiber wiring board, linearportions where the optical fibers are routed to be linear on thesubstrate and curved portions where the optical fibers are bent androuted to be curved. In order to prevent the increase in the bendingloss of the optical fiber in the curved portion, the curved portionneeds to be formed to have a minimum radius of curvature, specifically,a radius of curvature greater than a minimum radius of curvatureallowable with respect to the bending loss. However, in order to formthe curved portion with the radius of curvature greater than the minimumradius of curvature, the area of the substrate needs to beproportionally increased. Consequently, the size of the optical-fiberwiring board is enlarged.

[0005] In addition, on the optical-fiber wiring board, crossoversections are formed in each of which two optical fibers are routed suchthat the one optical fiber (upper optical fiber) crosses over the otheroptical fiber (lower optical fiber) routed in contact with the surfaceof the substrate. In this crossover section, the upper optical fiber isrouted to be bendable. In order to reduce the bending loss of the upperoptical fiber, the upper optical fiber needs to be routed to have aradius of curvature greater than the minimum radius of curvature.However, when the upper optical fiber is routed to have a radius ofcurvature greater than the minimum radius of curvature, mutualinterference occurs between the upper optical fiber and other opticalfibers routed adjacent to the lower optical fiber routed on thesubstrate. In order to prevent the interference, the wiring density ofthe optical fibers on the substrate needs to be reduced. In order toachieve the reduction, however, the area of the substrate needs to beenlarged, which consequently leads to enlargement of the size of theoptical-fiber wiring board.

[0006] When the number of the crossover sections formed on theoptical-fiber wiring board is reduced, the bending loss on the overalloptical-fiber wiring board is reduced. However, the reduction in thenumber of the crossover sections cannot be implemented unless otherwisethe area of the substrate is enlarged. As such, the size of theoptical-fiber wiring board needs to be enlarged to reduce the number ofthe crossover sections.

[0007] Thus, with ordinary communication-dedicated optical fibers usedwith a device such as an optical-fiber wiring board on which the fibersare bent and routed, the bending loss is increased. When the opticalfibers are routed considering the bending loss to prevent the problem,another problem arises in that the size of the device using the opticalfibers is enlarged.

[0008] The present invention is made in view of the above-describedcircumstances, and an object thereof is to provide an optical fibersuitable for use in an environment including many portions where theoptical fiber is bent. More particularly, the present invention is toprovide an optical fiber that is optimal for use with an optical-fiberwiring board, and an optical-fiber wiring board and an optical-fiberwiring assembly that use the optical fiber.

DISCLOSURE OF INVENTION

[0009] In order to achieve the above-described object, the presentinvention has been accomplished by paying attention to the fact that anoptical fiber used with an optical-fiber wiring board or the like has anoverall length of about 0.5 to 1 m, which is considerably small incomparison with the length of an optical fiber used for communication.Specifically, the present invention has been accomplished by payingattention to the fact that dissimilar to an ordinarycommunication-dedicated single mode optical fiber, such as acommunication-dedicated single mode optical fiber according to the ITU(International Telecommunication Union) standards, since the length ofthe optical fiber used with the aforementioned optical-fiber wiringboard or the like is short, the dispersion need not be taken intoconsideration as a design parameter.

[0010] In specific, a first aspect of the present invention is intendedfor an optical fiber of a single mode type including a core and acladding.

[0011] As characteristics, a relative refraction index difference of thecore and the cladding is increased to be greater than a relativerefraction index difference of a communication-dedicated single modeoptical fiber, and a core diameter is increased to be larger than a corediameter of the communication-dedicated single mode optical fiber.Thereby, a mode field diameter is set to be substantially the same as amode field diameter of the communication-dedicated single mode opticalfiber.

[0012] By thus increasing the relative refraction index difference ofthe core and the cladding to be greater than the relative refractionindex difference of the communication-dedicated single mode opticalfiber, confinement of light into the core is enhanced. Consequently,when the optical fiber is bent, it is suppressed that light in the coretransmit to the cladding. As such, in comparison to thecommunication-dedicated optical fiber, the optical fiber reduces thebending loss. That is, the minimum radius of curvature of the opticalfiber is less than the minimum radius of curvature of thecommunication-dedicated optical fiber.

[0013] However, when the relative refraction index difference of theoptical fiber is increased to be greater than that of thecommunication-dedicated single mode optical fiber, the mode fielddiameter thereof becomes smaller than the mode field diameter of thecommunication-dedicated single mode optical fiber. As such, when theoptical fiber is connected to the ordinary communication-dedicatedsingle mode optical fiber, the connection loss is increased.

[0014] For this reason, according to the optical fiber of the firstaspect of the present invention, the core diameter is increased to belarger than the core diameter of the communication-dedicated single modeoptical fiber, whereby the mode field diameter is set to besubstantially the same as the mode field diameter of thecommunication-dedicated single mode optical fiber. This arrangementprevents the connection loss from being increased when the optical fiberis connected to the communication-dedicated single mode optical fiber.

[0015] As described above, according to the optical fiber of the firstaspect of the present invention, the bending loss is reduced, and theconnection loss is not increased even when it is connected to thecommunication-dedicated single mode optical fiber. As such, the opticalfiber is suitable to an environment including many portions where it isbent; and it can be optimally used with, for example, an optical-fiberwiring board.

[0016] It is noted that the first aspect of the present invention isrealized by not considering the dispersion as an optical-fiber designparameter. For example, for the optical fiber to be used with a lengthof 10 m or shorter, the dispersion need not be taken into considerationas a design parameter.

[0017] Herein, the relative refraction index difference Δ [%] ispreferably set to satisfy:

Δ₀<Δ≦Δ₀+0.5[%],

[0018] where the relative refraction index difference of thecommunication-dedicated single mode optical fiber is represented by Δ₀[%].

[0019] The setting is thus performed for the following reasons.Generally, when the relative refraction index difference Δ is increased,a cut-off wavelength is increased. However, the cut-off wavelength needsto be set to be less than a wavelength (for example, 1.3 μm or 1.55 μm)of propagation light that propagates through the optical fiber. Hence,when the relative refraction index difference Δ is set to be excessivelylarge in comparison with the ordinary relative refraction indexdifference Δ₀, control of the cut-off wavelength to a predeterminedvalue becomes difficult. For this reason, the setting is preferablyperformed so that the deviation between the relative refraction indexdifference Δ of the optical fiber and the relative refraction indexdifference Δ₀ of the communication-dedicated single mode optical fiberis within 0.5%.

[0020] The cut-off wavelength herein is a theoretical cut-off wavelengththeoretically calculated according to the structure of the opticalfiber. In comparison, a practical cut-off wavelength (effective cut-offwavelength) is less than the theoretical cut-off wavelength dependingon, for example, the length of the optical fiber and the construction ofother optical fibers connected to the front and the back of the opticalfiber.

[0021] As such, the theoretical cut-off wavelength of the optical fibermay be set in such a manner that the effective cut-off wavelength isequal to or less than a wavelength of the propagation light.

[0022] For example, the theoretical cut-off wavelength λc [μm] may beset to satisfy:

λ<λc≦λ+0.05 [μm],

[0023] where the wavelength of the propagation light is represented by λ[μm].

[0024] Thus, even with the optical fiber designed by setting thetheoretical cut-off wavelength to be longer than the wavelength of thepropagation light, the effective cut-off wavelength becomes less thanthe wavelength of the propagation light. On the other hand, even withthe relative refraction index difference increased corresponding to theincrease in the length of the theoretical cut-off wavelength, the modefield diameter of the optical fiber takes a desired value. Consequently,an optical fiber for which the bending loss is even more reduced can beobtained.

[0025] A second aspect of the present invention is intended for anoptical-fiber wiring board including: an optical fiber of a single modetype including a core and a cladding; and a substrate on which theoptical fiber is routed.

[0026] As characteristics of the above, the optical fiber is constructedin such a manner that a relative refraction index difference of the coreand the cladding is increased to be greater than a relative refractionindex difference of a communication-dedicated single mode optical fiberand a core diameter thereof is increased to be larger than a corediameter of the communication-dedicated single mode optical fiber.Thereby, a mode field diameter thereof is set to be substantially thesame as a mode field diameter of the communication-dedicated single modeoptical fiber.

[0027] The optical-fiber wiring board may include a curved portion wherethe optical fiber is routed to be in a circular arc shape.

[0028] In addition, the optical-fiber wiring board may include acrossover section where two optical fibers are routed to cross over oneanother on the substrate.

[0029] The substrate preferably has an adhesive layer for adhering theoptical fiber.

[0030] The optical fibers on the optical-fiber wiring board may berouted on the substrate in such a manner as to perform matrix conversionof inputs of m ports (m represents a natural number) and n channels (nrepresents a natural number) into outputs of n ports and m channels.

[0031] As described above, since the optical fiber routed on thesubstrate of the optical-fiber wiring board has the relative refractionindex difference that is greater than the relative refraction indexdifference of the communication-dedicated single mode optical fiber, thebending loss is reduced. Therefore, the bending loss is not increasedeven when the optical fiber is routed at a small radius of curvature onthe substrate. The bending loss is not increased also in the crossoversection where the two optical fibers cross over one another.Consequently, miniaturization can be implemented for the optical-fiberwiring board. Further, since the minimum radius of curvature formed whenthe optical fiber is routed is small, a fiber-routing pattern of theoptical fiber on the optical-fiber wiring board can be changed to a morecomplex fiber-routing pattern.

[0032] Further, since the bending loss of the optical fiber is reduced,the performance of the optical-fiber wiring board is stabilized.Furthermore, for example, even in a case where the optical fiber routedon the substrate is bent following the substrate itself that is bent,the bending loss of the optical fiber is not increased. As such, theoptical-fiber wiring board is capable of securely maintaining apredetermined performance.

[0033] Preferably, the optical fiber on the optical-fiber wiring boardis constructed in such a manner that a relative refraction indexdifference Δ [%] thereof is set to satisfy:

Δ₀<Δ≦Δ₀+0.5 [%],

[0034] where the relative refraction index difference of thecommunication-dedicated single mode optical fiber is represented by Δ₀[%].

[0035] A theoretical cut-off wavelength of the optical fiber ispreferably set in such a manner that an effective cut-off wavelength isequal to or less than a wavelength of propagation light.

[0036] The theoretical cut-off wavelength λc [μm] of the optical fiberis preferably set to satisfy:

λ<λc≦+0.05 [μm],

[0037] where the wavelength of the propagation light is represented by λ[μm].

[0038] A third aspect of the present invention is intended for anoptical-fiber wiring assembly constructed in such a manner that acommunication-dedicated single mode optical fiber is connected to eachof input and output ports of an optical-fiber wiring board including anoptical fiber of a single mode type including: a core and a cladding;and a substrate on which the optical fiber is routed.

[0039] As characteristics of the above, the optical fiber is constructedin such a manner that a relative refraction index difference of the coreand the cladding is increased to be greater than a relative refractionindex difference of the communication-dedicated single mode opticalfiber and a core diameter thereof is increased to be larger than a corediameter of the communication-dedicated single mode optical fiber,whereby a mode field diameter thereof is set to be substantially thesame as a mode field diameter of the communication-dedicated single modeoptical fiber.

[0040] As described above, the mode field diameter of the optical fiberand the mode field diameter of the communication-dedicated single modeoptical fiber in the optical-fiber wiring assembly are substantially thesame. Therefore, even when the communication-dedicated single modeoptical fiber is connected to the optical fiber, the connection loss isnot increased. As such, an ordinary communication-dedicated single modeoptical fiber can be connected to each of input and output ports of theoptical-fiber wiring board.

BRIEF DESCRIPTION OF THE DRAWINGS

[0041]FIG. 1 is an exploded perspective view showing an optical matrixconversion wiring board according to an embodiment of the presentinvention.

[0042]FIG. 2 is a view showing an example of a sub-wiring board.

[0043]FIG. 3 is an enlarged cross-sectional view showing a crossoversection of the sub-wiring board.

[0044]FIG. 4 is a view showing the relationship among core diameters,mode field diameters and cut-off wavelengths.

[0045]FIG. 5 is a view showing fiber parameters of an ordinarilycommunication-dedicated single mode optical fiber, and examples of fiberparameters of optical fibers according to the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

[0046]FIG. 1 shows an example of an optical matrix conversion wiringboard 100 according to an embodiment of the present invention. Theoptical matrix conversion wiring board is an optical-fiber wiring boardthat converts inputs of m ports and n channels (hereinafter, referred toas “(m, n) inputs”. It is noted that the letters “m” and “n” eachrepresents a natural number) into outputs of n ports and m channels(hereinafter, referred to as “(n, m) outputs”). The optical matrixconversion wiring board 100 (hereinafter, referred to as an m×n opticalmatrix conversion wiring board) has advantages in that a number ofoptical fibers are arranged in units of a channel, thereby preventingerroneous wiring, and in that the optical fibers are routed on a plane,whereby a reduced storage area suffices for installation.

[0047] The optical matrix conversion wiring board 100 shown in FIG. 1includes 16 input terminals and 16 output terminals, and it isconstructed to be a 16×16 optical matrix conversion wiring board forconverting (I to XVI, 1 to 16) inputs to (1 to 16, I to XVI) outputs. InFIG. 1, input terminals and output terminals are partly omitted forillustration. The optical matrix conversion wiring board 100 isconstructed of first to forth sub-wiring boards 200 to 500 laminated oneabove the other.

[0048] As shown in FIG. 2, each of the sub-wiring boards 200 to 500(first sub-wiring board 200 in FIG. 2) is constructed of a substrate 210and optical fibers 220 routed on the substrate 210. In addition, asshown in FIG. 1, each of the sub-wiring boards 200 to 500 includesinput-side optical connectors 231 to which the input terminals areconnected and output-side optical connectors 241 to which the outputterminals are connected. The input-side optical connectors 231 and theoutput-side optical connectors 241 are individually formed of8-conductor MT connectors. On each of the sub-wiring boards 200 to 500,since 64 optical fibers 220 are routed, eight input-side opticalconnectors 231 and eight output-side optical connectors 241 are providedon each of the sub-wiring boards 200 to 500. The optical connectors 231and 241 are not limited to the 8-conductor connectors, but may be othermulti-conductor connectors.

[0049] In the optical matrix conversion wiring board 100, (I to VIII, 1to 8) inputs and (1 to 8, I to VIII) outputs are connected on the firstsub-wiring board 200, and (I to VIII, 9 to 16) inputs and (9 to 16, I toVIII) outputs are connected on the second sub-wiring board 300. Inaddition, (IX to XVI, 1 to 8) inputs and (1 to 8, IX to XVI) outputs areconnected on the third sub-wiring board 400, and (IX to XVI, 9 to 16)inputs and (9 to 16, IX to XVI) outputs are connected on the fourthsub-wiring board 500. According to the above-described construction, onthe optical matrix conversion wiring board 100, the (I to XVI, 1 to 16)inputs are fed to the individual input-side optical connectors 231 ofthe fist to fourth sub-wiring boards 200 to 500 via the input terminals.In addition, the (1 to 16, I to XVI) outputs are obtained from theindividual output-side optical connectors 241 of the first to fourthsub-wiring boards 200 to 500 via the output terminals.

[0050] Hereinafter, the structures of the individual sub-wiring boards200 to 500 will be described in detail with reference to FIGS. 2 and 3.The first to fourth sub-wiring boards 200 to 500 have same structureswith one another, except that the fiber-routing patterns of the opticalfibers 220 are different. As such, hereinafter, description will begiven of the first sub-wiring board 200 as an example.

[0051] The first sub-wiring board 200 includes the substrate 210 onwhich an adhesive layer 211 is formed, and the optical fiber 220 isadhered to the adhesive layer 211 and is thereby fixed to the substrate210.

[0052] A material having a high resistance against vibration ispreferably used for the substrate 210 to prevent the routed opticalfiber 220 from easily being deflected. As a material for forming thesubstrate 210, for example, a polyimide resin, a polyethyleneterephthalate resin or a polyethylene naphthalate resin may be used.

[0053] The adhesive layer 211 is a layer having tackiness or adhesionproperty, and it may be any layer capable of securely fixing the opticalfiber 220. The adhesive layer 211 may therefore be formed using a knownpressure sensitive adhesive or an adhesive (such as a siliconeadhesive).

[0054] In the sub-wiring board 200 thus formed, the optical fibers 220routed on the substrate 210 may be covered by a laminate layer (notshown). This enables the optical fibers 220 to be protected from outsideforces and humidity. In addition, the fiber-routing stability isenhanced, thereby improving the reliability. As the laminate layer,polytetrafluoroethylene (PTFE) or the like may be used.

[0055] In addition, on the first sub-wiring board 200, eight opticalfibers 220 are connected to each of the input-side optical connectors231, and each optical fiber bundle 232 is formed such that the eightoptical fibers 220 are routed in close contact with one another.Accordingly, eight input-side optical fiber bundles 232 are provided.

[0056] Each of the input-side optical fiber bundles 232 is routed in apredetermined fiber-routing pattern on the substrate 210 in a portionfrom an input-side edge portion of the substrate 210 to the vicinity ofa central portion thereof. Specifically, in a portion from an input-sideedge portion of the substrate 210 to the vicinity of a central portionthereof, each of the optical fiber bundles 232 is routed on thesubstrate 210 in such a manner as to form linear portions extending inthe X direction and the Y direction and curved portions 251 to 254(radii of curvature R1 to R4) curved to be a ¼ circular arc shape.

[0057] In the vicinity of the central portion of the substrate 210, theoptical fibers 220 constituting the each of the optical fiber bundles232 are separated one by one. At this time, each of the optical fibers220 is routed in such a manner as to form a curved portion 255 having aradius of curvature R5.

[0058] In the vicinity of the central portion of the substrate 210, theindividual optical fibers 220 separated from the one optical fiberbundle 232 are routed in close contact with individual optical fibers220 separated from seven other optical fiber bundles 232. Thereby, eightoutput-side optical fiber bundles 242, each of which includes eightoptical fibers 220 as one bundle, are formed. In this manner, theinput-side optical fiber bundles 232 individually corresponding tochannels (ch 1 to 8) including ports I to VIII are restructured into theoutput-side optical fiber bundles 242 individually corresponding tochannels (ch I to VIII) including ports 1 to 8. For example, each of theoptical fibers 220 of the input-side ch 1 is included in the ports 1 ofthe output-side ch I to VIII.

[0059] Also similar to the input-side optical fiber bundle 232, each ofthe output-side optical fiber bundle 242 is routed in a predeterminedfiber-routing pattern on the substrate 210 in the portion from thevicinity of a central portion of the substrate 210 to an output-sideedge portion thereof. More specifically, in a portion from the vicinityof a central portion of the substrate 210 to an output-side edge portionthereof, each of the optical fiber bundles 242 is routed on thesubstrate 210 in such a manner as to form linear portions extending inthe X direction and the Y direction and curved portions 256 to 258(radii of curvature R6 to R8) curved to be a ¼ circular arc shape. Inaddition, the output-side optical connectors 241 are individuallyconnected to the eight output-side optical fiber bundles 242.

[0060] The first sub-wiring board 200 may preferably be manufacturedusing, for example, an X-Y plotter (not shown). Specifically, afiber-routing head of the X-Y plotter is moved along the predeterminedfiber-routing pattern on the substrate 210, and concurrently, theoptical fiber 220 is provided onto the substrate 210 (adhesive layer211) via the fiber-routing head. At this time, the provided opticalfiber 220 is pressed at a predetermined force onto the adhesive layer211, and the optical fiber 220 is thereby adhered to the adhesive layer211. Thus, the optical fiber 220 is routed in the predeterminedfiber-routing pattern on the substrate 210. These steps are repeatedlyperformed for all the 64 optical fibers 220, thereby enabling the firstsub-wiring board 200 as shown in FIG. 2 to be manufactured.

[0061] The first sub-wiring board 200 (optical matrix conversion wiringboard 100) includes, for example, many crossover sections 259 in each ofwhich two optical fibers 220 routed in such a manner as to cross overone another on the substrate 210, and a number of curved portions 251 to258 in each of which the optical fiber 220 is routed in such a manner asto be curved to be the ¼ circular arc shape.

[0062] As shown in FIG. 3, a lower optical fiber 220 a is adhered ontothe adhesive layer 211 in the crossover section 259. On the other hand,an upper optical fiber 220 b is disposed in contact with the loweroptical fiber 220 a and to cross over the lower optical fiber 220 a. Inaddition, the upper optical fiber 220 b is disposed in such a manner asto have a predetermined radius of curvature Ra. The radius of curvatureRa is set to be greater than the minimum radius of curvature allowablewith respect to the bending loss of the optical fiber 220.

[0063] Similarly, in the individual curved portions 251 to 258, each ofthe radii of curvature R1 to R8 of the individual optical fibers 220 isset to be greater than the minimum radius of curvature. Each of thecurved portions 251 to 258 need not necessarily be in the circular arcshape, and the optical fiber 220 may instead be formed in such a manneras to have a radius of curvature greater than the minimum radius ofcurvature.

[0064] Hereinafter, the optical fibers 220 used with the optical matrixconversion wiring board 100 will be described. In comparison to anordinarily communication-dedicated single mode optical fiber, theoptical fiber 220 is shorter in the overall length. As such, dissimilarto the communication-dedicated single mode optical fiber, the opticalfiber 220 is designed without considering dispersion as a designparameter.

[0065] In specific, for the optical fiber 220, a relative refractionindex difference Δ [%] of a core and a cladding and a core diameterthereof are set in such a manner that a mode field diameter (MFD) and acut-off wavelength λc individually take predetermined values.

[0066] That is, as shown in FIG. 4, the core diameter, the MFD, thecut-off wavelength λc, and the relative refraction index difference Δ,which are design parameters of the optical fiber, are related to oneanother. When the cut-off wavelength λc and the MFD are set, therelative refraction index difference Δ and the core diameter areindividually determined according thereto. For example, as shown in FIG.5, in the case of a communication-dedicated single mode optical fiberaccording to the ITU standards, when the wavelength of propagated lightis 1.3 μm, the cut-off wavelength λc is determined to be 1.1 to 1.28 μm,and the MFD is determined to be 9.5±0.5 μm (G652). Thereby, the relativerefraction index difference Δ of the optical fiber is determined to be0.36%, and the core diameter thereof is determined to be 8 μm. When thewavelength of propagated light is 1.55 μm, an ordinarycommunication-dedicated single mode optical fiber is designed to have anMFD of 10.5±0.5 μm (it is noted that this value is a standard value, nota specification value). Accordingly, the relative refraction indexdifference Δ is determined to be 0.36%, and the core diameter isdetermined to be 8 μm.

[0067] In comparison to the above, the optical fiber 220 is designed tohave a relative refraction index difference Δ greater than a relativerefraction index difference (hereinafter, referred to as an ordinaryrelative refraction index difference Δ₀ [%]) of the normalcommunication-dedicated single mode optical fiber. Thereby, the opticalfiber 220 is imparted with an enhanced confinement of light into thecore. Consequently, even when the optical fiber 220 is bent, it issuppressed that light in the core transmit to the cladding. That is, theoptical fiber 220 reduces the bending loss.

[0068] Generally, when the relative refraction index difference Δ isgreater than the ordinary relative refraction index difference Δ₀, theMFD of the optical fiber is less than the MFD of the ordinarycommunication-dedicated single mode optical fiber (hereinafter, referredto as an ordinary MFD). In this state, when the optical fiber isconnected to the ordinary communication-dedicated single mode opticalfiber, the connection loss is increased. For this reason, the corediameter of the optical fiber 220 is increased to be larger than thecore diameter of the ordinary communication-dedicated single modeoptical fiber. Thereby, the MFD of the optical fiber 220 is set to besubstantially the same as the ordinary MFD, and the increase in theconnection loss is prevented.

[0069] In addition, generally, when the relative refraction indexdifference Δ is increased, the cut-off wavelength λc of the opticalfiber is increased. However, the cut-off wavelength λc needs to be setto be less than a wavelength (for example, 1.3 μm or 1.55 μm) ofpropagation light that propagates through the optical fiber. Hence, whenthe relative refraction index difference Δ is set to be excessivelygreat in comparison with the ordinary relative refraction indexdifference Δ₀, control of the cut-off wavelength to a predeterminedvalue becomes difficult. For this reason, the relative refraction indexdifference Δ is preferably set so that the deviation thereof withrespect to the ordinary relative refraction index difference Δ₀ iswithin 0.5%. Specifically, the relative refraction index difference Δ ispreferably set to satisfy:

Δ₀<Δ≦Δ₀+0.5 [%].

[0070] The cut-off wavelength λc is a theoretical cut-off wavelength λctheoretically calculated according to the structure of the opticalfiber. In comparison, a practical cut-off wavelength (effective cut-offwavelength) is less than the theoretical cut-off wavelength λc dependingon, for example, the length of the optical fiber (fiber-routing patternof the optical fiber 220 in each of the sub-wiring boards 200 to 500)and the construction of other optical fibers connected to the front andthe back of the optical fiber. As such, the theoretical cut-offwavelength λc of the optical fiber 220 is preferably set so that theeffective cut-off wavelength is equal to or less than thepropagation-light wavelength. In this case, while the effective cut-offwavelength is equal to or less than the propagation-light wavelength,the theoretical cut-off wavelength λc is increased to be greater thanthat in the ordinary case. Accordingly, even with the relativerefraction index difference Δ increased corresponding to the increase inthe length of the theoretical cut-off wavelength λc, an optical fiberwith a desired MFD can be designed. Consequently, an optical fiber forwhich the bending loss is even more reduced can be obtained.Specifically, the theoretical cut-off wavelength λc may be set to begreater than the propagation-light wavelength so that the deviationbetween the theoretical cut-off wavelength λc and the propagation-lightwavelength is 0.5 μm or less. More specifically, the theoretical cut-offwavelength λc may be set to satisfy:

λ<λc≦λ+0.05 [μm],

[0071] where the propagation-light wavelength is represented by λ [μm].

[0072] Design of the optical fiber 220 will now be described in detailwith reference to FIG. 4. An optical fiber used as a reference is anordinary communication-dedicated single mode optical fiber of which thepropagation-light wavelength is 1.55 μm. The relative refraction indexdifference Δ of this optical fiber is 0.36%, and the core diameterthereof is 8 μm (see arrow a in FIG. 4), whereby the cut-off wavelengthλc is set about 1.28 μm, and the MFD is set about 10.2 μm.

[0073] First, only the relative refraction index difference Δ isincreased to 0.38%. Thereby, the MFD is reduced to be slightly less than10 μm (see arrow b in the same figure). In this state, since the loss ofconnection to an ordinary communication-dedicated optical fiber isincreased, the MFD needs to be increased to an ordinary MFD.Specifically, the core diameter is increased to, for example, 9 μm,whereby the MFD is changed to 10.2 μm (see arrow c in the same figure)so as to become substantially the same as the ordinary MFD.

[0074] At this time, the cut-off wavelength λc is about 1.48 μm, whichis less than the propagation-light wavelength, and the optical fiber canbe used as an optical fiber of a 1.55 μm band.

[0075]FIG. 5 shows examples of fiber parameters of an optical fiber towhich the present invention is applied. Specifically, in a case wherethe relative refraction index difference Δ is set to 0.39% and the corediameter is set to 9.3 μm, an optical fiber having a cut-off wavelengthλc of 1.55 μm and an MFD of 10.5 μm can be obtained (Embodiment 1). Inaddition, in a case where the relative refraction index difference Δ isset to 0.425% and the core diameter is set to 8.8 μm, an optical fiberhaving a cut-off wavelength λc of 1.54 μm and an MFD of 10.0 μm can beobtained (Embodiment 2). In the cases of optical fibers to which thepresent invention is applied, the relative refraction index differencesΔ are 0.36% to 0.45%, and the core diameters are 7 μm to 10 μm.

[0076] Generally, the less the relative refraction index difference Δand the smaller the core diameter of an optical fiber, the more likelythe bending loss of the optical fiber occurs. However, in the case ofthe optical fiber according to the present invention, the absence ofconsideration of the dispersion enables design to be performed at adegree of freedom increased thereby. Specifically, increasing therelative refraction index difference Δ of the optical fiber enables thebending loss thereof to be reduced. In other words, this enables areduction in the minimum radius of curvature formed when routing theoptical fiber. Accordingly, using the optical fiber enables the minimumradius of curvature (Ra) in the crossover section 259 of each of thesub-wiring boards 200 to 500 to be reduced, and concurrently enablingreduction of each of the radii of curvature (R1 to R8) in the curvedportions 251 to 258 (see FIGS. 2 and 3). This enables the sizes of thesub-wiring boards 200 to 500 to be miniaturized, consequently resultingin miniaturization of the size of the optical matrix conversion wiringboard 100. In addition, the fiber-routing pattern of the optical fiberson the optical matrix conversion wiring board 100 can be changed to amore intricate fiber-routing pattern. Further, the reduction in thebending loss of the optical fiber 220 enables the performance of theoptical matrix conversion wiring board 100 to be stabilized.

[0077] Furthermore, even when the relative refraction index difference Δis large, the MFD of the optical fiber 220 is substantially the same asthe ordinary MFD. This enables the prevention of increase in connectionloss occurring when ordinary communication-dedicated optical fibers areconnected to the individual input and output ports of the optical matrixconversion wiring board 100.

[0078] It is noted that the optical fiber according to the presentinvention, that is, the optical fiber of which the bending loss isreduced and the MFD is set to be substantially the same as that of anordinary communication-dedicated single mode optical fiber, can suitablybe used not only for optical-fiber wiring boards, but also for other useenvironments including many portions where the fiber is bent.

1. An optical fiber of a single mode type comprising a core and acladding, wherein a relative refraction index difference of the core andthe cladding is increased to be greater than a relative refraction indexdifference of a communication-dedicated single mode optical fiber and acore diameter is increased to be larger than a core diameter of thecommunication-dedicated single mode optical fiber, whereby a mode fielddiameter is set to be substantially the same as a mode field diameter ofthe communication-dedicated single mode optical fiber.
 2. An opticalfiber according to claim 1, wherein a relative refraction indexdifference Δ [%] is set to satisfy: Δ₀<Δ≦Δ₀+0.5 [%], where the relativerefraction index difference of the communication-dedicated single modeoptical fiber is represented by Δ₀ [%].
 3. An optical fiber according toclaim 1, wherein a theoretical cut-off wavelength is set in such amanner that an effective cut-off wavelength is equal to or less than awavelength of propagation light.
 4. An optical fiber according to claim3, wherein a theoretical cut-off wavelength λc [μm] is set to satisfy:λ<λc≦λ+0.05 [μm], where the wavelength of the propagation light isrepresented by λ [μm].
 5. An optical-fiber wiring board comprising: anoptical fiber of a single mode type including a core and a cladding; anda substrate on which the optical fiber is routed, wherein the opticalfiber is constructed in such a manner that a relative refraction indexdifference of the core and the cladding is increased to be greater thana relative refraction index difference of a communication-dedicatedsingle mode optical fiber and a core diameter thereof is increased to belarger than a core diameter of the communication-dedicated single modeoptical fiber, whereby a mode field diameter thereof is set to besubstantially the same as a mode field diameter of thecommunication-dedicated single mode optical fiber.
 6. An optical-fiberwiring board according to claim 5, wherein a relative refraction indexdifference Δ [%] of the optical fiber is set to satisfy: Δ₀<Δ≦Δ₀+0.5[%], where the relative refraction index difference of thecommunication-dedicated single mode optical fiber is represented by Δ₀[%].
 7. An optical-fiber wiring board according to claim 5, wherein atheoretical cut-off wavelength of the optical fiber is set in such amanner that an effective cut-off wavelength is equal to or less than awavelength of propagation light.
 8. An optical fiber according to claim7, wherein the theoretical cut-off wavelength λc [μm] of the opticalfiber is set to satisfy: λ<λc≦λ+0.05 [μm], where the wavelength of thepropagation light is represented by λ [μm].
 9. An optical-fiber wiringboard according to claim 5, comprising a curved portion where theoptical fiber is routed to be in a circular arc shape.
 10. Anoptical-fiber wiring board according to claim 5, comprising a crossoversection where two optical fibers are routed to cross over one another onthe substrate.
 11. An optical-fiber wiring board according to claim 5,wherein the substrate has an adhesive layer for adhering the opticalfiber.
 12. An optical-fiber wiring board according to claim 5, whereinthe optical fibers are routed on the substrate in such a manner as toperform matrix conversion of inputs of m ports (m represents a naturalnumber) and n channels (n represents a natural number) into outputs of nports and m channels.
 13. An optical-fiber wiring assembly constructedin such a manner that a communication-dedicated single mode opticalfiber is connected to each of input and output ports of an optical-fiberwiring board including: an optical fiber of a single mode type having acore and a cladding; and a substrate on which the optical fiber isrouted, wherein the optical fiber is constructed in such a manner that arelative refraction index difference of the core and the cladding isincreased to be greater than a relative refraction index difference ofthe communication-dedicated single mode optical fiber and a corediameter thereof is increased to be larger than a core diameter of thecommunication-dedicated single mode optical fiber, whereby a mode fielddiameter thereof is set to be substantially the same as a mode fielddiameter of the communication-dedicated single mode optical fiber.